Inconel 738 alloy is a precipitation-reinforced, cast, nickel-based superalloy reinforced with γ′-Ni₃(Al, Ti) phase. The alloy owns a high volume fraction of the γ′-reinforced phase and has excellent creep resistance, strength, corrosion resistance, and good microstructure stability at high temperatures, rendering it a key material for aeroengine turbine blades. Using conventional methods of casting, forging, and machining, the complex structures of aeroengine turbine blades—which have multiple holes and thin walls—face high processing challenges and require long processing cycles. Recently, selective laser melting (SLM) technology has been especially useful for high precision and rapid manufacturing of complex thin-walled structures—including medium and small overhangs, complex internal cavities, and profiles. Thus, it serves as a new method for manufacturing aeroengine turbine blades. However, for many conventionally cast Inconel 738 alloy components, the extremely high-temperature gradients (≥10⁵ K/cm), rapid cooling rates (≥0.01 m/s), and cyclic thermal effects inherent to SLM technology can form cracks during SLM processing and subsequent heat treatments. The laser energy density plays a key role in determining the behavior of the molten pool formed from the nickel-based high-temperature alloy powder layer during the SLM process, which in turn influences the microstructure and mechanical properties of the formed aeroengine parts. Thus, the goal of this study is to investigate the effects of scanning speed and laser power on the densification of this alloy within the optimal range of laser energy densities, address the internal defects in additively manufactured Inconel 738 alloy, and optimize the parameters of the forming process.

Herein, we employed SLM technology as the preparation method and investigated the effects of laser power and scanning speed on densification of the SLMed Inconel 738 alloy. We used a one-factor experimental design and fabricated 50 Inconel 738 blocks. We analyzed the blocks using optical microscopy (OM) and calculated their densification using ImageJ software. The results reveal that—within a selected range of the printing parameters—an optimal interval of energy density exists that affects the densification of the fabricated alloy. In particular, higher scanning speeds lead to lower densification within this optimal interval, and the influence of the laser power on the densification aligns with that of the energy density. We then selected five blocks for mechanical property analyses, which were used to establish the relationship between the energy density, laser power, scanning speed, and the mechanical properties of the alloy. We determined the optimal processing parameters—i.e., those that resulted in the highest densification and the best mechanical properties—from these relationships.

Our analysis of the effects of different energy densities demonstrates that the densification of the Inconel 738 alloy initially increases and then decreases as the laser energy density increases during the SLM process. We find that the optimal range of laser energy densities is between 60 and 70 J/mm3. However, within a similar range of energy densities, the densification of the printed alloy samples displays fluctuations with substantial variations (Fig. 8). Moreover, as the scanning speed increases, the densification of the Inconel 738 blocks continuously decreases (Fig. 9). The relationship between laser power and densification follows a trend similar to that of the energy density, where the densification first increases and then decreases. In particular, peak densification occurs at an energy density (E) of 66.67 J/mm3 (Fig. 9). The samples obtained with a scanning speed of 1100 mm/s and laser powers of 260 W and 270 W exhibit similar energy densities and densifications. A comparison of the mechanical property curves for these two samples shows that their yield strengths, tensile strengths, and elongations are nearly identical. This indicates that—within the optimal range of energy densities—the laser power does not have a remarkable impact on densification. The samples obtained using a laser power of 270 W and scanning speeds of 1100 mm/s and 1150 mm/s exhibit similar energy densities, although the densification obtained with the scanning speed of 1100 mm/s (99.92%) is slightly higher than that obtained with 1150 mm/s (99.91%). The yield strength and elongation of the sample fabricated at a scanning speed of 1100 mm/s are considerably higher than those of the sample fabricated at 1150 mm/s. This demonstrates that, within the optimal range of energy densities, increased scanning speed adversely affects the mechanical properties, which is consistent with its effect on densification. The highest densification was observed in the samples fabricated with a scanning speed of 950 mm/s and a laser power of either 270 W (99.95%) or 260 W (99.97%); these were the two samples selected for mechanical property testing. We find that the yield strength of the sample fabricated with a laser power of 260 W and scanning speed of 950 mm/s is 563.199 MPa and that its tensile strength is 1446.412 MPa, both slightly higher than those of the sample fabricated with a laser power of 270 W and a scanning speed of 950 mm/s. The elongation of the sample fabricated with a laser power of 260 W and scanning speed of 950 mm/s reaches 9.2%, corresponding to the highest energy of plastic deformation. The sample found to have the optimal mechanical properties is the one fabricated with a laser power of 260 W, scanning speed of 950 mm/s, and pitch of 0.10 mm.

Herein, we used scanning electron microscopy, OM, and tensile testing to investigate the influence of the laser energy density on the densification, metallurgical defects, and mechanical properties of the SLM-formed high-temperature alloy Inconel 738. Based on the optimal laser energy density, we further analyzed the effects of the laser power, scanning speed, and pitch on the densification of the samples. Accordingly, we draw the following conclusions from these experimental results:

(1) The laser energy density plays a critical role in the densification of the SLM-formed Inconel 738 alloy samples. The densification initially increases sharply and then decreases gradually as the laser energy density increases. The maximum densification of Inconel 738 is approximately 99.97% at E=68.42 J/mm³.

(2) Although the optimal laser energy density for the SLM-formed Inconel 738 alloy is determined, other different parameters will still affect the densification under the same laser energy density. This occurs because the energy density is influenced by a combination of factors, including the laser power (P), scanning speed (v), and pitch (s).

(3) After determining the optimal energy density, the effect of the laser power on the densification is still not clear. The densification of the sample decreases as the scanning speed increases. Given a fixed laser energy density, the densification of the sample can be modified by adjusting the scanning speed.

(4) For the SLM-formed Inconel 738 alloy, the printing parameters that yield the optimal mechanical properties are as follows: E=68.42 J/mm³, P=260 W, v=950 mm/s, and s=0.10 mm.